docs: def _iter_subplots(self):
          """Yield (i, j, dim)."""
          for i in range(self.n_cols):
              for j in range(self.n_cols):
                  # Skip lower-diagonal subplots.
                  if i > j:
                      continue
                  dim = self.grid_dim[i][j]
                  dim_x, dim_y = dim.split(',')
                  yield i, j, dim_x, dim_y